FIG. 1 shows a conventional disk drive 10 which has a hard disk assembly ("HDA"). The HDA includes at least one magnetic disk ("disk") 12 having a plurality of concentric data tracks, a spindle motor 13 for rapidly rotating the disk 12, and a head stack assembly ("HSA") 20.
The HSA 20 includes a transducer head 140 for reading and writing the data onto the magnetic disk 12. The typical HSA 20 further includes an actuator assembly 30 and a head gimbal assembly ("HGA") 100. The head gimbal assembly (HGA) 100 extends from the actuator assembly 30 and biases the head 140 towards the disk 12. The industry presently favors a "rotary" or "swing-type" actuator assembly that move the transducer head 140 in a generally radial manner across the rotating disk 12.
A servo control system moves the actuator assembly 30 to controllably position the transducer head in order to read or write information from or to particular tracks on the disk 12. A flex circuit on the side of the actuator assembly carries read and write signals between a controller 14 and the head 140.
Each data track includes embedded servo data sectors and user data sectors that are alternately located around the track. The servo data comprises track ID fields for identifying data tracks and servo bursts (e.g. A, B) for defining burst pair centerlines.
The system reads the track ID and samples the servo bursts to generate a position error signal (PES) relative to a desired position. The servo system uses the magnitude of the PES to move the actuator assembly 30 of the HSA 20 in order to drive the PES toward zero and thereby achieve or maintain the desired position of the transducer head 140.
In operation, the servo control system moves the transducer head 140 toward a desired data track during a "seek" mode based on the track ID field. Once the transducer head 140 is over the desired track, the servo control system enters a "track follow" mode and uses the servo bursts to keep the transducer head at a desired fractional track position relative to a burst pair centerline.
Older disk drives use an inductive transducer which both reads and writes data onto the disk 12. An inductive transducer drive is usually track following directly on a burst pair centerline except, perhaps, when bumped off track or when settling. Even then, an inductive transducer is usually about 80% of the data track pitch in width and is generally uniformly sensitive across its 80% width such that it can operate .+-.40% away from the burst pair centerline. An inductive head drive, therefore, often uses a quadrature pattern of four 100% (of track pitch) servo bursts which provides 50% spacing between two- burst pair centerlines.
Newer disk drives use magnetoresistive heads ("MR heads") which have two separate transducers: a magnetoresistive transducer for reading and an inductive transducer for writing. MR heads advantageously support increased a real data density, but suffer from having: (1) an inherent physical separation between the read and write transducers; (2) a smaller average ratio between the width of the read transducer and the data track pitch (about 66% instead of 80%); (3) a relatively wide variance in the width of read transducers about the average (ranging from less than 50% to nearly 100%) which range must often be accommodated due to manufacturing limitations and cost constraints; and (4) a non-linear magnetic response characteristic across the width of the read transducer.
The separation between the read and write transducers causes them to be variably skewed or radially displaced relative to the burst pair centerlines as they are moved from the inside diameter to the outside diameter of the disk. This skew creates a track following problem because positioning is determined by reading the servo bursts with the read transducer even when writing data with the radially displaced write transducer. Because of this skew, the servo control system must read or write while track following away from the burst pair centerline. This process is known as "jogging" and is implemented as a fractional track offset from a servo burst pair centerline. The servo system usually jogs when reading but not when writing because a read operation can always be repeated whereas a write error may cause irreversible damage to neighboring data tracks. In other words, the required jogging offset depends on position for reading, and is always zero for writing. The required jogging offset is usually determined from a lookup table.
As just explained, there is a frequent need to jog the MR read head. The available jogging range, however, is limited by the physical width of the read transducer. As noted above, the magnetoresistive read transducer is relatively narrow, being on average about 66% of a data track pitch wide as compared to 80% for an inductive transducer. The maximum jogging range is one half the physical width, such that for a 66% transducer it is .+-.33%. A particular read transducer may be even narrower than average, being as narrow as 50% of a track pitch or less. In the case of a 50% transducer, the maximum jogging range is .+-.25%. Finally, the read transducer's nonlinear response characteristics may further reduce the available jogging range. Under some circumstances of uncompensated nonlinearity, therefore, it may be possible to jog a 50% read transducer only .+-.20% from a given burst pair centerline before it is necessary to commutate to an adjacent burst pair centerline.
The jogging requirement coupled with the width and linearity related limitations have made it difficult to use a magnetoresistive read transducer with a conventional quadrature pattern of 100% servo bursts because the burst pair centerlines are separated by 50% of a track pitch and only suited, therefore, for a read transducer which can be jogged by at least .+-.25%. Some members of the drive industry have addressed this problem by using narrow servo bursts to create more closely spaced burst pair centerlines so that a narrow read transducer could effectively commutate to a second adjacent burst pair centerline before being jogged more than 20% away from a first burst pair centerline. One such servo pattern uses four servo bursts that are 2/3 of a track pitch in width and are arranged in quadrature to provide a 1/3, 1/3, 1/3 pattern of burst pair centerlines. Unfortunately, a 1/3, 1/3, 1/3 pattern is relatively expensive in terms of manufacturing because each disk drive must spend more time in the servo track writer.